Rainwater – Pump Size

One of the reasons why most rainwater systems use too much power is because because the pump is too big. For other reasons see Rainwater – Pump Issues

Most pump suppliers and design charts will specify a larger pump than you really need (They sell a more expensive pump, It’s less likely that the pump will be returned because it isn’t big enough, and they aren’t paying the power bill)

If you want to pick the most economic pump here is how to go about it.

Instantaneous Flow

What is the most flow you need at any one time. Here are some figures for typical house fittings.

Fitting

Flow Litres/min

Tap

10 to 15

Tap with Flow Restrictor

4 to 6

Low Flow Shower

7 to 9

Washing Machine*

4 to 10

Dishwasher

4 to 6

Toilet

3 to 5

Garden Sprinkler**

10 to 15

*To get to the lower figure you will need to close the supply valve this will add a few minutes to the wash but will help with issues like Water Hammer.

**It’s really better to irrigate the garden with a separate pump.

Add together the highest flow rate fittings that you think you will want to run together, which will give you a Total Flow Rate ‘Q’

Pressure

You need to aim for a pressure at the fitting of around 150kPa (15m of Head)

To get this pressure you need to:

Measure the height of the furthest fitting above the lowest level in the tank ‘Hs)

Calculate the Pressure Loss ‘Hf‘ due to Friction in the pipes See table below